Datasheet AD8235 (Analog Devices) - 17

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Data Sheet AD8235 +VS +VS AD8235 AD8235 REF REF –VS –VS TRANSFORMER TRANSFORMER +VS +VS C C 1 R fHIGH-PASS = AD8235 2πRC AD8235 REF REF R –VS –VS
38 0 1-
AC-COUPLED AC-COUPLED
821 0 Figure 40. Creating an IBIAS Path
INPUT BIAS CURRENT RETURN PATH RF INTERFERENCE
The AD8235 input bias current is extremely small at less than RF rectification is often a problem in applications where there are 50 pA. Nonetheless, the input bias current must have a return large RF signals. The problem appears as a small dc offset voltage. path to common. When the source, such as a transformer, The AD8235, by its nature, has a 3.1 pF gate capacitance, CG, at cannot provide a return current path, one should be created each input. Matched series resistors form a natural low-pass filter (see Figure 40). that reduces rectification at high frequency (see Figure 41). The relationship between external, matched series resistors and the
INPUT PROTECTION
internal gate capacitance is expressed as All terminals of the AD8235 are protected against ESD. In addition, 1 the input structure allows for dc overload conditions a diode drop FilterFreq  DIFF 2 RC π above the positive supply and a diode drop below the negative G supply. Voltages beyond a diode drop of the supplies cause the 1 FilterFreq  ESD diodes to conduct and enable current to flow through the CM 2 RC π G diode. Therefore, an external resistor should be used in series
+VS
with each of the inputs to limit current for voltages above +VS. In either scenario, the AD8235 safely handles a continuous 6 mA
0.1µF 10µF
current at room temperature.
SDN
For applications where the AD8235 encounters extreme overload voltages, as in cardiac defibrillators, external series
+IN
resistors and low leakage diode clamps, such as BAV199Ls,
R C
FJH1100s, or SP720s, should be used.
G V AD8235 OUT –VS R CG REF –VS –IN 0.1µF 10µF
39 -0
–V
11
S
82 0 Figure 41. RFI Filtering Without External Capacitors Rev. A | Page 17 of 20 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION CONNECTION DIAGRAM PIN CONFIGURATION REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION BASIC OPERATION GAIN SELECTION SHUTDOWN FEATURE LAYOUT RECOMMENDATIONS Pad Opening Pad Type Pad Finish Board Thickness Grounding REFERENCE TERMINAL POWER SUPPLY REGULATION AND BYPASSING INPUT BIAS CURRENT RETURN PATH INPUT PROTECTION RF INTERFERENCE COMMON-MODE INPUT VOLTAGE RANGE APPLICATIONS INFORMATION AC-COUPLED INSTRUMENTATION AMPLIFIER LOW POWER HEART RATE MONITOR OUTLINE DIMENSIONS ORDERING GUIDE